It is known that the proper alignment of the steerable wheel of a vehicle plays an important part in the steering performance, road handling characteristics and tire wear. Wheel alignment is designed specifically to meet the requirements of a given vehicle and thus a manufacturer has different alignment specifications for each different model of vehicle. In the manufacture of a vehicle, great care is taken to establish the steerable wheel alignment according to design specifications and such alignment should be maintained during the life of the vehicle. However, certain aspects of alignment are difficult to measure or otherwise determine from the vehicle itself.
As is well understood by those skilled in the art, the front wheels of a car are typically supported in a geometry that is especially designed to enhance certain attributes of car performance such as road handling and tire wear. Included in this geometry of the front wheels are the features known as camber, caster, toe and steering axis inclination (SAI). These features all have values which may be expressed in terms of an angular quantity. The car manufacturer specifies a design or nominal value for each of such features and, in some cars, the value may be zero for a given feature, and may be different for the left and right wheels of the car. The values of these features for a given car should conform to those specified by the manufacturer. Accordingly, as a matter of quality control the manufacturer typically measures or otherwise checks these features. In some cases it may be part of an audit procedure of the manufacturer and in some cases adjustments are made as required.
This invention is addressed to the determination of the caster angle; and in connection with that, the camber angle and SAI angle must also be considered. The caster angle is the forward or rearward tilt of the steering axis. The steering axis is an imaginary line drawn through the center of the kingpin or through the centers of the upper and lower ball joints or other such pivot members. The camber angle is the inward or outward tilt of the wheel relative to a vertical plane. The SAI angle is the angle formed between the steering axis and a vertical line, as viewed from the front of the vehicle. These aspects of the geometry of the front wheel alignment result in a tendency of a point of tire contact with the ground (road) to change in response to steering rightward or leftward from the straight ahead direction.
The camber angle and the toe angle are readily measurable on the vehicle and it is common practice to use camber and toe measuring apparatus in the vehicle factory as well as for service and maintenance during the life of the vehicle. These values are measured between the plane of the wheel and a horizontal or vertical plane with the vehicle on a horizontal surface and such planes are readily accessed on the vehicle for measurement. However, the caster angle is by definition the tilt of the steering axis of the wheel in a vertical plane and it cannot be readily accessed on the vehicle because it is an imaginary line drawn between two points such as the upper and lower ball joints. Similarly, the SAI angle is the angle of an imaginary line, the steering axis, relative to the vertical. The determination of these angles presents a special problem and there has been a longstanding need for a method for accurate determination for quality control and manufacture of the vehicle as well as subsequent maintenance.
A prior art method and apparatus for determining caster trail of a steerable wheel on a vehicle is described in Merrill U.S. Pat. No. 3,793,734 granted Feb. 26, 1974. The apparatus of this patent includes a pair of fore-and-aft rollers which support the steerable wheel at two points of reference. The swinging of the wheel away from a straight ahead direction is said to cause a force to be applied to the rollers parallel to the longitudinal axis thereof. According to the patent, the force distributed along each roller is measured and compared, one with the other, to compute static, kinetic and pneumatic caster trail. In one embodiment, the patent describes the measurement of the reference points along the transverse axis of the vehicle with the wheel steered right through a given steering angle and also with the wheel steered left through the same angle. From this information, the caster trail of the wheel is calculated.
A general object of this invention is to provide an improved method and apparatus for determining the caster angle and the SAI angle of a steerable wheel on a vehicle and to overcome certain disadvantages of the prior art.